The incidence of capsular contracture is higher in lupus overall using implants. If you really don't have the condition, but a positive ANA and a history of symptoms then I'd move forward with the surgery. If your rheumatologist is telling you that you have the condition and clearly are simply awaiting the next lupic symptom to come up then I'd be much more concerned with capsular contracture, because I believe the incidence is higher in your patient population.
Overall, the treatment for lupus can decrease a capsule formation, but typically the control of lupic symptomatology and general systemic immune response ocurrs in waveform (good control, then not so good control). You could very well develop a capsule at some point in that time.
The transaxillary approach requires a quite large incision for a gel implant. It's not a great idea that I usually discourage because someone is losing sight of the benefit of this and other approaches. I've done it before and it is possible without much struggle, it just makes more sense to do it via IMF or PA approaches.
The Mentor and Inamed statistics on contracture rates are available for your review at their specific sites for thier 3 and 6 year studies but your case will likely fall into a patient demographic (lupus) that was excluded from these studies.
Pick a board certified (ASPS) plastic surgeon in your community and discuss these issues in detail to come to a sound decision and conclusion.